1. Field of the Invention
The present invention relates generally to image recording apparatuses such as printers for recording image on a recording medium, and more particularly, to image forming apparatuses having a plurality of printing heads.
2. Description of the Related Art
With reference to FIG. 8, a conventional image recording apparatus, for example, a dot matrix type thermal printer, includes a platen roller 73 around which printing paper 72 is wound and a head unit 70 for printing image on the printing paper 72. The head unit 70 includes a printing head 71, an ink ribbon 74 and reels 75 and 76.
The platen roller 73 is rotatably supported by a frame (not shown). The printing paper 72 is wound around the circumferential surface of the platen roller 73. The printing head 71 is provided in proximity to the platen roller 73. The printing head 71 is rectangular in shape with a surface opposing to the circumferential surface of the platen roller 73 on which surface a plurality of heating elements (for example, 24 elements) are arranged linearly along a rotation direction (sub-scanning direction) of the platen roller 73. Each heating element has the same shape and each distance between adjacent heating elements is also the same. Each heating element is heated in a short time period when a voltage is applied thereto discharges heat in a short time period when no voltage is applied.
The two reels 75 and 76 are disposed in proximity to the printing head 71. The ink ribbon 74 slides on the surface of the printing head 71 on which the heating elements are provided to be fed from the reel 75 to the reel 76. When a heating element of the printing head 71 is heated, ink on the ink ribbon 74 in contact with the heating element is melted and attached to the printing paper 72. The head unit 70 including the printing head 71, the ink ribbon 74 and the reels 75 and 76 is capable of reciprocating in parallel with the axis of the platen roller 73, and the image is formed on the printing paper 72 by using the printing head 71 while the head unit 70 is reciprocating.
In a conventional image recording apparatus, the respective heating elements disposed on the printing head 71 have substantially the same shape. Therefore, each pixel of image recorded by the printing head 71 has the same area. Thus, solid printing (painting out a predetermined region in fixed concentration), for example, requires a long period of time to form image. Solid-printing of an area much larger than a pixel area of image formed by a heating element produces, for example, blank (a part of a solid-printed region has a concentration lower than that of the remaining part of the region). A pixel of an increased area formed by the heating element enables image formation in a short time period, while reducing defective image such as blank. However, resolution of the formed image is deteriorated.